Casing boundary layers in a multistage axial compressor

Abstract

The casing boundary layers strongly influence the performance of axial?flow compressors. Computational theories exist for calculating casing?wall boundary?layer growth, and although they are relatively successful in helping to describe overall compressor behaviour, they are less useful in predicting the actual details of the boundary layer. These theories contain assumptions regarding the extent of collaterality in the boundary layer, the difference in blade force within and outside the layer, etc. More information on the actual boundary layer is necessary to validate these assumptions, especially in situations where the extent of the boundary layer is not small compared to the blade height. This thesis describes an experimental investigation of a casing boundary layer in a specially built four?stage axial compressor. The compressor had a tip diameter of 720 mm and a hub?to?tip ratio of 0.6. The blading was designed for a nominal degree of reaction of 0.6, unvarying from hub to tip. Arrangements were made to produce a specially thick boundary layer at the entry to the compressor. Extensive measurements were made of the velocity profiles behind the moving and stationary blade rows for four flow coefficients. Fluid?stress and pressure measurements were also made over the same flow range. The measurements have been used to estimate quantities such as blade?force defects, wall?shear stresses, etc., which are necessary for use in computational methods for casing?wall boundary?layer growth. The experimental flow has also been assessed in comparison with the assumptions inherent in the calculation methods.